Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 221119
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
719 AM EDT Sun Jul 22 2018


.AVIATION...

Plentiful low-level moisture will remain entrenched beneath a
shallow inversion around 4 kft that is expected to strengthen with
weak cold air advection in breezy northeast flow. Broad low pressure
will continue slowly moving southeast into northern Ohio Valley, but
secondary low pressure will track northwest from the Mid-Atlantic
region towards Lake Huron, bringing continued VCSH with intermittent
-SHRA. Thunderstorm chances look very meager with moist adiabatic
profile and will leave mention out of the TAFs at this time. Main
concern will be variable ceilings yet again today, ranging from at
times low-end MVFR to VFR. Brief periods of IFR are not out of the
question, especially at KPTK and KFNT where hi-res signal is highest
for more persistent shower activity today. Northeast winds will gust
15-20 knots at times before weakening and becoming more northerly
overnight.

For DTW...Challenging ceiling forecast for the entire period, but
high confidence that the bulk of the period will feature ceilings
below 5 kft. Best shot at -SHRA appears to be in the 13z-20z
timeframe. Northeast runway operations will be preferred with breezy
northeast flow through at least 00z.

.DTW THRESHOLD PROBABILITIES...

* High in ceilings at or below 5000 ft entire period, although
  periodically ceilings will mix out above 5000 ft.

* Very low in thunderstorms occurrence today.

&&

.PREV DISCUSSION...
Issued at 401 AM EDT Sun Jul 22 2018

DISCUSSION...

Large upper level low pressure system continues to plague the region
this morning with the center of the cyclonic circulation now
arriving over the Ohio River in southwest Ohio. Surprise of the
overnight has been the weak, lake effect thunderstorm activity that
has pushed off of Lake Erie and impacted Monroe and a small portion
of Wayne counties. Not a whole lot going with stable near surface
environment, but heavy rainfall has been observed.

In response to low pressure pushing south, flow trajectories
will become increasingly northeasterly this morning allowing cooler
and more drier low level air to infiltrate the Thumb/northern cwa
prior to 18Z, the southern cwa between 18-21Z this afternoon. Yet
another wrinkle in what has been an arduous system to attempt at
details. The big ticket item is the PA edge wave/compact low
pressure system that will continue to the northwest and backdoor
moisture back into the cwa. Sure has been interesting to watch the
evolution of the NWP the past few days with this system. Model data
shows the compact absolute vorticity center now penetrating directly
within the heart of the barotropic low. Models do in fact show a
well developed, coherent 700mb circulation that will pivot across
and down into portions of SW Lower Michigan by tonight. Given degree
of saturation that currently exists within the parent cyclone and
the strength of midlevel system relative isentropic ascent that is
forecasted, preference is to side very pessimistic with the forecast
today. While there will be some lower level dry air that exists this
morning, any precipitation that begins out of the middeck is not
expected to evaporate. Prob space is showing very strong positive
trends in precipitation today between 10-22Z. Interesting part is
only pinging those probs for .10 or less. Narrative is overcast, and
plenty of light rain chances for much of today. The most
organized activity will be along the Lake Huron shoreline and the
Thumb. Most likely see some drizzle activity there as well. Lowered
temperatures a good amount, ranging in the upper 60s to 70s.

A large amount of blocking over North America will cause the upper
level low to become elongated and move very little during the early
part of the weak. A better portion of the absolute vorticity energy
is expected to become sheared out and stall directly overhead of
Southeast Michigan on Monday. As a result, expecting more chances
for shower activity with perhaps a diurnal component. Westerly flow
should increase aloft by Tuesday but the low level pressure gradient
should remain weak. The question becomes where does the moisture
axis line out. ECMWF is more bullish with moisture locally, while
the nam is to the east. Kept precipitation chances in the forecast
for Tuesday. A little uneasy with the forecast highs for Tuesday,
but MOS consensus exists for middle 80s.

MARINE...

Broad low pressure will continue to drift southeastward into the
northern Ohio Valley today. As this low drifts southeast, a
secondary area of low pressure will move northwest from the Mid-
Atlantic region towards Lake Huron. This low pressure will stall and
weaken heading into Monday. Unsettled weather featuring periodic
showers will continue through today, with drier conditions
attempting to work into the region starting Monday. Moderate
northeast flow will prevail today, with gusts generally confined to
15-25 knots outside of western Lake Erie, where frequent gusts
around 25 knots will pose hazardous conditions for small craft. The
flow will weaken beginning Monday as weak surface ridging and
slightly drier weather builds into the Great Lakes.

HYDROLOGY...

Intermittent showers and possibly a few rumbles of thunder will
continue today across much of the region as two areas of broad low
pressure hover near lower Michigan. Rainfall amounts today will
generally be under a quarter of an inch, with localized amounts up
to a half of an inch possible where heavier showers or thunderstorms
develop. No flooding concerns are anticipated outside of minor
ponding of water on roadways.

&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Beach Hazards Statement through this evening for MIZ083.

Lake Huron...NONE.
Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory until 10 PM EDT this evening for LEZ444.

&&

$$

AVIATION.....IRL
DISCUSSION...CB
MARINE.......IRL
HYDROLOGY....IRL


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